This invention relates to a filter circuit and, more particularly, to a filter circuit which includes a charge transfer device, such as a bucket brigade device.
Transversal filters, both recursive and non-recursive, have been formed of charge transfer devices, such as bucket brigade devices. In such a device, an input signal generally is supplied to a charge storage device, such as a capacitor, which is responsive to a clock signal supplied thereto to be charged to a level corresponding to a sampled level of the input signal. Thereafter, and in response to a next-following clock pulse, a switching element, such as a transistor, is rendered conductive to transfer the charge which had been stored on the aforementioned capacitor to a similar capacitor, or other charge storage device, in a next-following stage. Then, still another switching element, such as another transistor, is rendered conductive to transfer the charge from that capacitor to a succeeding capacitor. This operation continues, whereby the initially sampled signal level is transferred from stage-to-stage through the charge transfer device. Of course, while the initial sampled signal level is transferred in this manner, successive samples of the input signal are obtained and transferred in sequence. The charge transfer device thus functions as a time delay circuit, whereby suitable analog samples of the input signal "ripple" through the device.
Such a charge transfer device of the aforementioned type, which also is known as a bucket brigade device, has been used as a non-recursive transversal filter. The voltages produced as a function of the charges which are stored in different predetermined stages of the bucket brigade device are "tapped", or derived, suitably weighted, and summed to produce an output signal. Typically, the voltages produced at such predetermined stages of the bucket brigade device are supplied via emitter-follower transistor circuitry to the weighting circuits and, thence, are summed at an output. While such emitter-follower circuitry serves to isolate the weighting and output circuitry from the stages of the bucket brigade device, such emitter-follower circuitry has a deleterious affect upon the charge transfer device. For example, the collector-base capacitance of each emitter-follower transistor tends to reduce the effective pulse height of the clock pulses which are supplied to the bucket brigade device and which are necessary to transfer samples of the input signal therethrough. Furthermore, the base current drawn by the emitter-follower transistors has an undesirable influence on the charge transfer efficiency of the bucket brigade device. As a consequence of such emitter-follower circuitry, the dynamic range of the signal which can be filtered by using the charge transfer device is reduced.
When the aforementioned bucket brigade device is used to form a recursive transversal filter, emitter-follower circuitry is used to supply the voltages produced at predetermined stages of the bucket brigade device to weighting circuits, from which the "tapped", weighted voltages are summed and then sampled-and-held and fed back to, for example, the input of the bucket brigade device. Since this recursive transversal filter uses emitter-follower transistors, it suffers from many of the same disadvantages noted hereinabove in connection with the non-recursive transversal filter. In addition, the use of emitter-follower transistors, weighting circuits, analog summing circuits and sample-and-hold circuitry results in a relatively complicated, and thus expensive, arrangement, and also consumes a significant amount of electric power. Still further, if an unexpected transient, such as a current spike, passes through the emitter-follower circuitry, or through the analog summing circuit, or through the sample-and-hole circuit, such a current spike may be superimposed thereby onto the power supply or onto ground potential. This will adversely affect the various stages in the bucket brigade device. As yet another difficulty in the aforementioned recursive transversal filter, the DC potential of the voltages supplied to the emitter-follower circuitry often is not equal to the DC potential of the voltage which is fed back from the sample-and-hole circuit. As a result thereof, the filter circuit may be unstable.